Characterization of Si in a W matrix Using Diffraction Contrast in the TEM

Abstract

The practice of using a Si based gas prior to W deposition is a process used within the semiconductor industry to aid in uniform and continuous filling of interlevel interconnects. SIMS has been effectively used to determine the concentration of Si in the bulk of a W nucleation layer formed during a blanket deposition. RBS has also been employed to evaluate the W:Si ratio in the same nucleation layer. Such techniques have proven effective in the detection of low levels of Si in W; however, the lateral spatial resolution of these techniques is orders of magnitude larger than the dimensions necessary to directly analyze modern integrated circuit devices. Furthermore, it is postulated that the material character observed in films produced by blanket deposition may have subtle but significant differences from the material deposited within the confines of a high aspect ratio via (i.e., 350-500 nm in diameter x 1000 nm deep.) Therefore, the TEM becomes the instrument of choice for the site-specific analytical characterization of such features. Although TEM is unquestionably capable of the requisite lateral spatial resolution, the analysis, more specifically the spectroscopic analysis, remains non-trivial. X-ray energy dispersive spectrometry (XEDS) techniques are severely hampered by the fact that the typical energy resolution of an XEDS detector is not sufficient to resolve the characteristic x-ray energy levels between the Si Kα (1.737 keV) and the W Mα (1.774 keV) [1,2]. Although the energy resolution of wavelength dispersive X-ray spectrometry (WDS) is sufficient to detect Si, the lateral spatial resolution can be problematic and more significantly, X-ray fluorescence from the surrounding SiO 2 matrix obscures the analysis. Electron energy loss techniques also are capable of the requisite energy resolution, but when the specimen is thinned sufficiently for the analysis the surrounding SiO 2 matrix has been observed to become mechanically unstable under the electron beam. Lastly, the microcalorimeter, which is a seemingly viable option, has not yet achieved widespread availability and may be subject to the same fluorescence artifacts as observed in WDS.By exploiting fundamental diffraction contrast mechanisms that are readily available in conventional TEM, a method to detect the presence of Si in W has been developed. The effect has been observed in both TEM and STEM modes of operation using both a Philips EM430 operating with a LaB 6 source at 300kV as well as a Philips/FEI Tecnai F30 operating with a field emission gun at 300 kV. In blanket films deposited using the Si gas process a distinct bright band of ~4nm is observed in dark field (DF) TEM images which are formed by allowing diffracted beams that correspond to lattice spacings less than 0.100nm to pass through the objective aperture. Any such band is systematically absent in analogously formed DF images of blanket films deposited without the introduction of the Si gas as shown in Figure 1. Figures 2a and 2b show corresponding bright field (BF) and DF TEM ima...

FIB Damage in Silicon: Amorphization or Redeposition?

“…Redeposition of sputtered atoms has also been reported as a result of FIB milling [4]. The propensity for redeposition increases when FIB milling is performed in a confined and/or a high aspect ratio trench, or when FIB conditions are used that contribute to factors that increase the sputtering rate (e.g., using a higher beam current) [5]. Observations have shown that FIB milling with Ga at an energy of 30 keV will produce amorphization damage along a Si side-wall that is ~ 28 nm thick and up to 20 wt% Ga may be present within the damage region [6].…”